The present invention relates to the art of heat sinks and cold plates and finds particular application in conjunction with electronic circuitry used in industrial variable-speed electric motor drives and will be described with particular reference thereto. However, it will be appreciated that the present invention will also find application in conjunction with other electronic devices including non-industrial electronic devices and in any other application which requires a heat transfer or exchange.
A. Drive Heat
It is well known that variable speed drives of the type used to control industrial electric motors include numerous electronic components. Among the various electronic components used in typical variable-speed drives, all generate heat to a varying degree during operation. Typically, high-power switching devices such as IGBTs, diodes, SCRs, capacitors and the like are responsible for generating most of the heat in a variable-speed drive.
It is also well known that, in addition to causing damage to electronic components, if rated device temperatures are exceeded, drive heat can affect the operating characteristics of devices and therefore may affect motor control. Generally the industry has approached varying drive temperatures in two distinct ways including heat sinking and adjustment of drive control to compensate for the effects of heat on device operation.
1. Control to Compensate for Drive Heat
With respect to drive control, the operating characteristics of many drive devices and of equipment which is controlled by the devices change as a function of temperature. For example, at a first temperature one PWM switching pattern may yield a first current through a stator winding while at a second temperature the same PWM pattern yields a second current through the winding wherein the first and second currents are different. To compensate for varying device operation, elaborate control systems have been designed which sense various system characteristics and, based thereon, modify device control signals. These systems are complex to design and are relatively expensive as the parameters to be controlled are typically several times removed from the feedback signals used to control the parameters.
2. Heat Sinks to Dissipate Drive Heat
With respect to heat sinks, most sinks are air cooled but recently several liquid cooled sinks have been developed and employed to increase heat dissipating capabilities. One such liquid cooled sink is described in U.S. patent application Ser. No. 09/009,441 ("the '441 sink") which was filed on Jan. 20, 1998, is entitled "Heat Sink Apparatus and Method for Making the Same", is commonly owned with this application and is incorporated herein by reference. The '441 sink includes a conduit construct within a sink body portion wherein the construct and body portion are each formed of either aluminum or copper. To form an exemplary '441 sink, a conduit construct is configured out of copper. To form complex constructs having many bends often pre-formed conduit segments are brazed together. After forming the construct, the construct is coated with a barrier material (e.g. a water based graphite silica coating on an electro-deposited coating of nickel) which blocks alloying between the construct and molten aluminum and is placed within a mold. Then, molten aluminum is poured into the mold around the construct and the aluminum is allowed to cool.
Molding processes can be grouped into two general categories including one-shot molding and permanent or reusable molding processes. In the case of one-shot molding, a rigid yet easily destructible mold form is constructed so that an internal surface defines external features of an item to be formed (hereinafter "the item"). With the form constructed, the form is filled with molten material which then hardens to form the item. Often one-shot molds are formed of sand which, after the molten material hardens to form the item, can be cracked apart to remove the item from the mold. The sand is then reused to construct another mold form and the process is repeated.
In the case of a permanent mold, a rigid, typically steel mold form is constructed having an internal surface which defines external features of the item to be formed. With the form constructed the form is filled with molten material which then hardens to form the item. Perm-mold cooling can be expedited by oil which operates as a heat transfer fluid during the molding process. Unlike a one-shot form, the permanent mold form (hereinafter "the perm-mold") is reusable. Thus, after the molten material hardens, perm-mold sections are separated and the item is removed. Then, the perm-mold sections are again arranged to form another item.
Because perm-molds are reusable, despite initial additional expense, perm-molds are often more economical. This is particularly true in cases where huge numbers of identical items have to be formed rapidly. In addition to being advantageous via reuse, because perm-mold cooling can be expedited via oil, using perm-molds can increase the speed with which the molding process can occur. For example, where it might take 45 minutes to cool a sand molded item, oil can typically be used to cool a perm-mold in less than one minute. For these reasons, where possible, it is usually desirable to use perm-molds instead of one-shot molds.
It has been recognized that in any molding process there may be several sources of pressure within the mold form which can damage an item being formed and can be dangerous. In particular, in cases where a copper conduit construct is placed in a mold form and molten aluminum is provided there around, there are three primary sources of form pressure including outgassing, hydrogen draw and water vaporization.
Outgassing occurs when the hot molten aluminum heats up the copper construct and the crystalline structure of the construct material changes giving off a gas.
Hydrogen draw occurs as the copper heats up and hydrogen is effectively drawn from within the conduit through the conduit wall and forced into the aluminum via the molten aluminum heat.
Water vaporization occurs where a water based material is used to form the alloy-blocking barrier between the conduit construct and the molten aluminum. In this case, if the water in the barrier material is not completely baked off prior to placing the construct in the form and filling the form with molten aluminum, the aluminum heat causes the water to vaporize and expand further increasing the gas and hence pressure in the mold form. In addition, because a mold is typically open to ambient conditions, vaporization may occur as a result of humidity in the tube and mold cavity prior to a pour.
In each of these cases, the gases which are released into the molten aluminum cause pressure within the form. Similar problems occur when the construct is aluminum and the molten material is copper or when a stainless steel construct is used.
Gas escaping into the molten aluminum through an alloy barrier material can cause a void in the barrier material thereby allowing a path for alloying between the molten aluminum and the copper conduit construct. The alloying causes "blow through" and blocks the conduit thereby rendering the sink useless.
In addition, gas escaping into the molten aluminum expands due to the aluminum heat increasing form pressure. If form pressure exceeds a maximum level, the form and molten material therein can explode.
Moreover, even where gas escaping into the molten aluminum does not cause an explosion, the gas may become entrapped in the aluminum and cause "dross" or voids within the sink body portion which result in less efficient heat dissipation. Often, to render a sink which includes voids useable, another process has to be performed whereby voids are identified within the sink, holes are drilled into the voids and then the voids and holes are filled with molten aluminum to eliminate the voids. Obviously this addition process increases sink costs.
To minimize the amount of gas escaping into the molten material, in the '441 sink the barrier layer between the conduit construct and the molten material includes nickel which acts as a "skin" to block gas from entering the molten material. While there are several advantages of using a nickel electroplate, there are two primary advantages. First, braze alloy has a solidus temperature of 1190.degree. F. where as the pour temperature of aluminum is approximately 1300.degree. F. Thus, the nickel plating prevents softening of the braze alloy and transfers heat to the adjacent copper. Second, the braze alloy includes sliver which is pyrophoric with aluminum. The nickel plating prevents sliver-aluminum interaction. In order for the nickel to operate or as gas barrier the nickel laden barrier has to be at least a minimal thickness along all points along the conduit construct. While the minimal thickness can be assumed in a controlled lab environment in a less controlled manufacturing environment barrier thickness may vary, and hence, while the nickel skin may block some gas, in many cases combined crystallization outgassing and hydrogen draw cause gas to pass through the nickel barrier into the molten aluminum.
Escaping gas is particularly problematic at brazed joints between conduit sections or segments. Typical brazing compound includes a copper-silver alloy which has a substantially lower melting temperature (e.g. approximately 791.degree. F.) than the copper conduit sections. Because of the lower melting temperature, the copper in the brazing compound recrystallizes at a lower temperature than the copper conduit and hence additional outgassing occurs at brazed joints.
There is yet another source of pressure which can occur in either one-shot or perm-mold processes which can be potentially dangerous and which is referred to as a "double-block". Imagine a conduit construct having an input end and an output end which is placed in a mold form, the form sealed around each of the input and output ends and each of the ends open. As aluminum is poured into the form, two blow throughs occur adjacent the two end so that air is trapped therebetween. As the air heats it expands and is forced through the conduit and into the form thereby increasing form pressure. Once again the form pressure may cause an explosion.
In the case of one-shot molding, sand molds are usually porous so that, within a relatively low pressure range, gas within the mold form escapes through the mold from walls. The nickel skin and escaping gas can often maintain form pressure below the maximum pressure range and therefore minimizes the possibility of causing an explosion using a sand mold form.
Nevertheless if the form pressure exceeds a maximum pressure an explosion is still possible using a sand mold. In addition, the sand mold does nothing to reduce the possibility of blow through.
In the case of a perm-mold, typical perm-molds are formed of steel and therefore are not porous. Thus, form pressure due to even a small amount of gas escaping into, and expanding in, the form can be extremely dangerous. Thus, despite the advantages associated with a reusable prem-mold, the industry has failed to develop a way to form an aluminum/copper heat sink using a perm-mold.
Thus, while perm-molding is desirable from a cost and efficiency perspective, gassing problems have prohibited perm-molding in the liquid cooled heat sink industry. In addition, gassing problems and the potential for explosion have reduced the desire to use one-shot molds in the liquid cooled heat sink industry.
Therefore, it would be advantageous have a method and an apparatus for reducing gassing problems so that molding process and more specifically perm-molding processes could be used to expedite heat sink manufacturing. In addition, it would be advantageous to have a method and an apparatus which could maintain drive temperature so that simpler drivers could be employed to control loads.